Abstract:The anoxic sediment of an acidic (pH similar to 3) iron- and sulfate-rich lake and its pore water was studied with respect to the turnover rates of solid and dissolved iron and sulfur species. High sedimentation rates of iron (570 g m-2 a-1) lead to an enrichment of the upper (0-5 cm) acidic sediment zone (pH < 4) with schwertmannite (Fe8O8)(OH)x(SO4)y (approximate to 350 g kg-1). Microbial iron-reduction rates measured by closed vessel incubation technique were highest close to the sediment-water interface (250 nmol cm-3) d-1), sulfate reduction measured by the 35S-tracer technique was not detectable in this zone. The absence of sulfide allowed complete reoxidation of dissolved Fe(II) diffusing into oxic parts of the lake water. Thus, an iron cycle is established where acidity generation through this process (1.0-4.7 mol m-2 a-1) balanced the alkalinity gain through microbial iron reduction in this zone (0.65-4.0 mol m-2 a-1). Predominance of iron over sulfate reduction under acidic conditions is further stabilized by the transformation of schwertmannite to goethite at a depth of 3-5 cm, which releases acidity at a rate of 3.5 mol m-2 a-1. Below, pore-water pH increased to values between 5 and 6, sulfate reduction occurred with a maximum rate of 14 nmol cm-3 d-1 at 9 cm depth. Release of Fe(II) and a short turnover time of reduced sulfur relative to the sediment age implies that most of the sulfide formed seemed to be recycled to sulfate at this depth, presumably coupled to the reduction of iron. Consequently, net alkalinity is generated at low rates only (0.12 mol m-2 a-1).